rate: replace flaky test in TestMeanProcessingDuration
rate: replace flaky test in TestMeanProcessingDuration

Testing the results of calcMeanDuration() is already done in
TestCalcMeanDuration. Depending on the accuracy of sleeps in programs
invites inconsistent behaviour and flakes, especially when running them
in high-load environments like CI VMs. What this really exercises is the
Go runtime and OS schedulers and the underlying virt/hardware.

Shorten the sleeps in the test cycles, check that a mean processing duration
was recorded (whatever its value may be), and add a test case to
TestCalcMeanDuration to compensate somewhat. Crafting more test cases is
tricky due to the API returning floats that are always off by fractions.

Signed-off-by: Timo Beckers <98c2d5a1e48c998bd9ba9dbc53d6857beae1c9bd@isovalent.com>

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